If Namco wasn't too busy being money hatted by Micro$oft then they would have found the solution that would have made their pockets and everybody happy. Released the game in 2008 for PS3 then ported the game for X360 and released it 2009. They would have gotten all those huge sales numbers from PS3 and also gotten the smidge of sales from X360. The fans would have loved them and they would have profited.
The trouble is, Micro$oft is too smart and too rich for the optimal solution to have been reached. Micro$oft probably paid extra just to delay the PS3 version from being released and also to make it so that both versions were equal. Don't you just love Micro$oft? Always fucking around with their competitor's customers.
Have you ever thought that the reason Namco made this decision was because they saw the sales parity of Soulcalibur IV and realized a one console approach with a late port to the other might be a mistake? It would seem to me if Microsoft was going to throw money at Namco they would have done what they did with Namco a dozen other times and get a year of exclusivity on the title. Isn't that what happened with Ace Combat, Katamari, Tales of Vesperia, Eternal Sonata, Culdcept Saga, etc.?
I can't imagine if a year delay and going multiplatform is what caused the fans to up and leave that Tekken ever had any fans at all. I don't expect Final Fantasy XIII to take a gigantic hit in sales because of it. Why? The series has fans. Fans stick around. I know it may come as difficult to believe, but maybe Tekken is just dying out and that's why it's selling like it is.
I never think about SCIV and Tekken comparisons, they're never spot on anyway. SC has been more of Namco's multiplatform offering. Anyway as you can see from the 1st week sales of both games, it is clear that fighting games is dying only in X360. Maybe you've considered that it's because of their controller? Nah you're right it must be the genre itself. It's not like SF4, KoF and BlazBlue sold significantly more on PS3 than in X360 right?
Being a PS3 owner, I can honestly say that I tend to view multiplatform games to be of lower quality than exclusive games. Not because I think PS3 is better but because the developers don't have to make them equal according to some clause that Micro$oft put in their fucking contract. (Pardon the french, I just hate Micro$oft's dirty business practices) With that said, I tend not to buy a game if it's multiplatform.
As for sales in general of this I am certain, if they released Tekken 6 with the game engine they had back in 2008 they would have moved PS3 hardware and would have sold significantly more over the period of time that it took to make the game multiplatform. I also bet it would have looked way better than the X360 version. Just look at the opening video for Tekken6:BR (included in the game) the video of the arcade version looks a lot better than the game itself.